Determination of Total hardness in Water

Determination of Total hardness in Water

AIM:

To determine the amount of total hardness present in a given sample.

PRINCIPLE:

Ethylene diamine tetra-acetic acid(EDTA) is an hexadentate ligand that forms a stable complex with hardness ions.the Erichrome black T (EBT) is a indicator that form wine red colour in the presence of hardness ions like calcium,magnesium,etc. when EDTA react with this wine red solution,the hardness ions that detach from EBT and attach to the EDTA and forms a complex.the EBT remains as blue colour that is the end point.

STEP-1: Metal ion(sample) + EBT(blue colour) -------> [Metal-EBT] wine red.

STEP-2:  [Metal-EBT] wine red + EDTA(colourless) ------->[Metal-EDTA]sky blue colour. 

APPARATUS REQUIRED:

1)Conicle flasks.

2)Pipette.

3)Burette.

4)Standard volumetric flasks.

5)Glass rod.

6)Beakers.

CHEMICALS REQUIRED:

1)Reagent grade water (Distilled water or millipore water).

2)Disodium salt of EDTA.

3)Magnesium salt of EDTA.

4)Ammonium chloride.

5)Liquor Ammonia.

6)Erichrome black-T.

7)Triethanolamine.

8)Calcium carbonate.

9)Hydrochloric acid.

REAGENTS AND PREPARATIONS:

1)Standard 0.01M EDTA solution:

         

The EDTA is not directly soluble in water so we use  Disodium salt of EDTA.

Dissolve 3.723g of Disodium salt of EDTA in reagent grade water containing beaker and make up to 1000ml mark in volumetric standard flask.shake well and lable it.

2)Ammonium buffer solution:

First solution: Dissolve 16.9g of Ammonium chloride Salt in 143 ml liquor ammonia.

Second solution: Dissolve 1.25g of Magnesium salt of EDTA in little amount of water.

Add the second solution to the first.mix well and make-up to 250 ml with reagent grade water.check the PH of this solution should be 10.

3)EBT indicator:

    

Dissolve 0.5g of EBT in 100 ml of Triethanolamine and store in a bottle.

4)Standard Calcium carbonate solution for standarization:

Dry little amount of Calcium carbonate in oven at 180 degrees for an hour.weight 0.1g and dissolve it with small amount of distilled water.add a drop of 1:1 HCL.then make up  to 100 ml using reagent grade water.

PROCEDURE:

 

Wash Conicle Flasks thoroughly with distilled water. Pipette out and add the 1 ml of buffer solution to buffer the sample PH to 10. add 2 drops of EBT indicator.it become blue colour.then add the sample aliquot.now the solution become wine red if the hardness metal ions are present or the Colour wine red that is directly proportional to the concentration of  hardness ions.immediately start titration against 0.01M disodium salt of EDTA solution taken in the Burette.add EDTA slowly and shake constantly.the end point is Sky blue colour.note the volume of EDTA consumed.

STANDARDIZATION OF 0.01 EDTA:

We preparing  the EDTA solution that is not the correct molarity for sometimes.molarity may differ by various factors like temperature,time,etc.to know the exact molarity of that EDTA solution.we have to do standardization.the procedure of standarization is same as told earlier.but we using here standard calcium carbonate solution instead of sample.take two clean conicle flasks,add 1 ml of buffer solution and 2 drops of EBT indicator.add 10 ml of standard calcium carbonate solution and titrate against 0.01M EDTA solution and note the volume.

Calculation of standardization:

                V1×M1/V2 = M2

V1 = Volume of calcium carbonate solution (10ml)

V2 =  Volume of 0.01M EDTA consumed.

M1 = Standard Molarity of EDTA(0.01)

M2 = The exact Molarity of present time.

Example:

 

Burette reading of 0.01M EDTA is 9.8 ml for 10ml calcium carbonate solution taken.so the Molarity will be

           10.0 × 0.01 / 9.8 = 0.0102M

SAMPLE CALCULATION:

Total hardness = (V2-V1) × M × 100 × 1000 / Sv 

V1 = Initial reading of burette.

V2 = Final reading of burette.

M = Molarity of EDTA solution from standardization. 

100 = Molecular weight of calcium carbonate. 

1000 = For milligram conversion

Sv = Sample volume.

Model calculation:

TH = (2.3-0) × 0.0102 × 100 × 1000 / 10 = 234.6 mg/l.

SAMPLING:

For saving the EDTA solution,take sample volume related to electrical conductivity as given below.

*Take 10ml of sample if the electrical conductivity of that sample is nearer to 1000 micro siemens per centimetre.

*Take 25ml of sample if the electrical conductivity of that sample is nearer to 700 micro siemens per centimetre.

*Take 100ml of sample if the electrical conductivity of that sample is nearer to 100 to 200 micro siemens per centimetre.

*If above 2000EC, 5ml or below is suitable.